A presently preferred implementation of the inventive subject matter described herein is especially suited for synchronization of an uplink time difference of arrival (U-TDOA) wireless location system, or a hybrid system employing U-TDOA and angle of arrival (AoA) location technologies. Such systems may be used in connection with wireless communication systems employing spread spectrum techniques and rely on the uplink radio path between a user equipment (UE) device in the active state and a UMS base station (Node B) for the collection of radio signals, which are then used for TDOA and/or TDOA/AoA location calculations.
Code Division Multiple Access (CDMA) is a now common method for transmission of voice and data over radio. TruePosition was a pioneer in location of CDMA mobiles when in the year 2000, it conducted extensive testing with Verizon Wireless in mid-town Manhattan, N.Y. Verizon Laboratories used the rigorous test plan published by the CDMA Development Group (CDG) to determine the performance of TruePosition's network-based location technology in the challenging urban canyon (10 to 25 story buildings) environment. The WLS demonstrated sub-100 meter location results in a variety of indoor, outdoor, pedestrian, and moving vehicle scenarios. In the trial, unmodified CDMA (IS-95) mobile phones were used to make more than 30,000 test calls. These calls were placed by both Verizon Labs (formerly GTE Labs) and TruePosition in an area covered by 30 cell sites hosting time difference of arrival (TDOA) receivers.
The inventive techniques and concepts described herein apply to code-division radio communications systems, including the technologies referred to in technical specifications as CDMAOne (TIA/EIA IS-95 CDMA with IS-95A and IS-95B revisions), CDMA2000 family of radio protocols (as defined by the 3rd Generation Partnership Project 2 (3GPP2)), and in the Wideband Code-Division Multiple-Access (W-CDMA) radio system defined by the 3rd Generation Partnership Project (3GPP) as part of the Universal Mobile Telephone System (UMTS). The UMTS model discussed herein is an exemplary but not exclusive environment in which the present invention may be used. FIG. 1 depicts exemplary UMTS environment in which the present invention may be employed. These are explained in greater detail below.
To date, the UMTS option using the Frequency Division Duplex (FDD mode) of Wideband Code Division Multiple Access (W-CDMA) as the underlying air interface has been most widely deployed. Frequency Division Duplex is employed in UMTS to provide an uplink and downlink radio channel between the network and the user. The uplink and downlink frequencies are assigned and use separate spectral bands. FDD UMTS transceivers must tune between the uplink and downlink frequencies to transmit and receive, respectively. W-CDMA is a direct sequence spread spectrum system where base stations are not synchronized. Asynchronous base stations and thus asynchronous radio signaling requires mobile devices to acquire a timing reference and to synchronize to a base station (a Node B in UMTS) before communications can commence. In a UMTS, FDD, W-CDMA-based, system, the mobile device receives the Broadcast Channel (BC) from the base station (called the Node B in UMTS) to acquire the rough timing needed to access the Reverse Access Channel (RACH). This acquisition and synchronization procedure is called a “cell search”.
UMTS Frame and Slot Synchronization
In a W-CDMA system, the primary and secondary synchronization downlink (Node B to UE) channels (P-SCH, S-SCH) provide radio frame and time slot synchronization. The basic unit of time in UMTS radio signals is a 10 millisecond (ms) radio frame, which is divided into 15 slots of 2560 chips each. UMTS radio signals from a Node B to a UE are “downlink signals,” while radio signals in the reverse direction are called “uplink signals.” This structure is depicted in FIG. 2 and explained in greater detail below.
For each UE, initial cell search algorithms are used to synchronize the UE to a Node B. The UE accomplishes this procedure via a common downlink channel called the physical synchronization channel (PSCH).
When a UE is first powered on, the UE performs a cell search. In the cell search, the UE looks first for a downlink synchronization channel (SCH). The SCH is a common downlink channel transmitted from the cell allowing UE's within the radio footprint of the cell to synchronize at the slot and frame levels and to determine the particular scrambling code group of the cell. As specified in technical specifications for the UMTS standards, the downlink synchronization channel (DL-SCH or just SCH) is a sparse downlink channel that is only active during the first 256 chips of each slot. The SCH is made up of two sub-channels, the Primary SCH (PSCH) and the Secondary SCH (SSCH). The PSCH 256 chip sequence, or PSCH code, is the same in all slots of the SCH for all cells. In contrast, the SSCH 256 chip sequence, or SSCH code, may be different in each of the 15 slots of a radio frame and is used to identify one of 64 possible scrambling code groups. In other words, each radio frame of the SCH repeats a scrambling code group sequence associated with the respective transmitting cell. Each SSCH code is taken from an alphabet of 16 possible SSCH codes.
As part of the cell search, the UE first uses the PSCH to achieve slot synchronization. In this regard, the UE correlates received samples of the received PSCH against the known PSCH 256 chip sequence (which is the same for all slots) and, based on the location of the correlation peak, determines a slot reference time. Once the slot reference time is determined, the UE is slot synchronized and can determine when each slot starts in a received radio frame.
After slot synchronization, the UE ceases processing of the PSCH and begins processing the SSCH. In particular, the UE correlates the particular sequence of 15 SSCH codes in a received radio frame against known sequences to achieve frame synchronization and to determine the scrambling code group of the cell. Identification of the scrambling code group then enables the UE to descramble all of the other downlink channels of the cell such as the Common Pilot Channel (CPICH)) necessary for UMTS voice/data communications to begin.
The now synchronized UE can then move to the active state and access the uplink Random Access Channel. The Random Access Channel (RACH) is an uplink transport channel. The RACH is always received from the entire cell. The RACH is characterized by a collision risk and by being transmitted using open loop power control. While on the RACH, the UE sends a long pilot sequence allowing the Node B to determine the UE's time alignment. Once the UE has moved to the conversation stage of a call and is assigned to a DPCCH, pilot sequences transmitted by the UE are used to maintain the timing alignment. A total of 3 to 8 bits per slot are used for the mid-call uplink pilot sequences with 15 (0 to 14) slots available per frame. (As known to those of skill in the field of wireless communications, the term “DPCCH” stands for Dedicated Physical Control Channel. The DPCCH is the physical channel on which the signaling is transmitted, both on the uplink by the UE to the Node-B base transceiver station and on the downlink by the Node-B to the UE.)
The purpose of the time slot structure in UMTS is to provide a timing framework for determining when various events can occur. For example, a user's data rate can change for every frame, and power control commands are sent every slot (thus giving WCDMA a power control rate of 1,500 Hz). The data in WCDMA is modified by both spreading and scrambling codes prior to transmission. De-scrambling and de-spreading the received spread spectrum signal requires accurate alignment of the received data to the de-scrambling/de-spreading codes. If the WLS is tasked mid-call via the Iub or LMS, and therefore has no knowledge of the RACH bursts made by the UE, and since the power control of the W-CDMA system precludes inexpensive broadcast channel monitoring, the WLS is presented with a problem in collecting uplink signals from the UE for location purposes. As explained below, the present invention addresses this problem.